Electrolytic plating or electrolytic surface treatment of metals has been conducted using an article to be treated as the cathode and using, as the counter electrode, a soluble anode or an insoluble anode comprising a corrosion-resistant material such as lead or a lead alloy.
Soluble anodes have conventionally been used extensively because, theoretically, electrolytic plating operations employing a soluble anode can be conducted continuously without changing the composition of the electrolyte solution, that is, while the same metal as that deposited on the cathode (from the electrolyte solution) is released from, and supplied by, the anode in an amount equal to the deposited amount. However, use of soluble anodes has generally been defective. For example, the balance between the anode and cathode is disturbed resulting in the need to regulate the composition of the electrolyte solution or frequently replace the anode with a fresh one. Thus, the maintenance of the electrolytic system is troublesome. Another problem associated with the use of soluble anodes is that the distance between the cathode and anode is not constant. Therefore, soluble anodes have been unable to meet the recent demand for higher quality, higher speed, and energy saving, and as a result, insoluble anodes which do not dissolve in electrolytic baths (which changes the composition thereof) and which can be treated independently of electrodes have come to be used.
Lead or a lead alloy is used as a material for such insoluble anodes. The lead-based anode has the merits of inexpensiveness and is easily shaped. However, the use of such lead-based anodes are problematic. For example, when electrolysis is conducted at a high current density, i.e., at a high speed, the electrode material dissolves into the electrolyte solution at a rate of several milligrams per W.H to contaminate the electrolyte solution, leading to poor product quality. Another problem exists if the electrolysis is continued further. In such a case, the lead or lead alloy itself softens resulting in impaired dimensional stability. Although a platinum-plated titanium electrode is also being used as an insoluble electrode (in addition to the lead-based electrode), it is expensive, and; disadvantageously, its life is greatly shortened if on-off operations are repeatedly conducted.
On the other hand, a so-called dimensionally stable electrode (DSE) was developed which comprises a valve metal substrate and, provided thereon, a coating mainly comprising an oxide of a platinum group metal. Use of this electrode, which is regarded as free from most of the conventional problems, is spreading rapidly. In particular, in the caustic soda-producing electrolysis not involving oxygen generation, which is the mainly employed technique in the present-day industrial electrolysis for caustic soda production, almost 100% of the electrolytic cells employ a DSE.
Hitherto, it has been attempted to use the abovedescribed DSE in electrolysis involving oxygen generation, and the use thereof is rapidly expanding in recent years. One example of a DSE is an electrode comprising a substrate made of titanium or a titanium alloy and, formed on the substrate, an oxide coating containing iridium as an electrode material, and titanium or tantalum as a stabilizer. The most serious problem associated with these kinds of electrodes has been that when used in oxygen-generating electrolysis, the electrode forms a passive-state layer at the interface between the coating and the substrate and becomes unable to be used any longer before the electrode material is completely consumed. As a result of studies by the present inventors, they succeeded in inhibiting the formation of such a passive-state layer by providing a thin layer of an electrically conductive oxide at the coating-substrate interface.
However, even in the case of such an insoluble metal electrode having a thin layer of an electrically conductive oxide, its life in oxygen-generating electrolysis is one year at the most when the electrode is used at a current density of about 100 A/dm.sup.2. This life is extremely short as compared to the lives of several years or more which the insoluble metal electrode has when used in soda-producing electrolysis. Therefore, it has been desired to increase the life of the insoluble metal electrodes used in oxygen-generating electrolysis.